A light receiving circuit, which is widely used in optical communication systems, includes an identifying and reproducing circuit that identifies and reproduces received light. First the identifying and reproducing circuit determines whether an amplitude of the pulse waveform of an electric signal into which an optical signal has been converted is greater or smaller than a specified threshold value (hereinafter referred to as the “threshold”). When the amplitude is smaller than the threshold, the identifying and reproducing circuit determines it as “0”. When greater than the threshold, the circuit determines it as “1”. Then the circuit reproduces a data signal indicating “0” (hereinafter referred to as “0 data”) or a data signal indicating “1” (hereinafter referred to as “1 data”) and output the data.
In general, data used by digital optical transmission systems of the backbone system are scrambled. Thus, the ratio of 1 data identified and reproduced by the identifying and reproducing circuit to 0 data identified and reproduced thereby is 1:1.
Unexamined Japanese Patent Application KOKAI Publications No. 2003-198475 (hereinafter referred to as “Patent Literature 1”) discloses a light receiving device including an identifying and reproducing circuit that uses the characteristics of scrambled digital data (pages 3 and 4 and FIG. 1).
The light receiving device as disclosed in Patent Literature 1 includes an identification threshold correction circuit. This identification threshold correction circuit corrects the threshold voltage set to the identifying and reproducing circuit so that the ratio of 1 data reproduced by the identifying and reproducing circuit to 0 data reproduced thereby is 1:1.
The waveform of an electric signal supplied to the identifying and reproducing circuit is not an ideal pulse waveform. It is distorted by noise caused by a photodiode, etc. Even when the threshold voltage is set so that the ratio of 1 data to 0 data is 1:1, the BER (Bit Error Rate) eventually increases to a level where it is not practically usable. The BER changes according to the intensity of light received by a light receiving circuit, waveform distortions thereof, and the like.
For example, a light receiving circuit including a photodiode and an amplifier with a high OSNR (optical signal-to-noise ratio) is used to receive an optical signal without waveform distortions. In this example, a heat noise of the amplifier is much greater than a shot noise generated by the photodiode. In this case, when the threshold voltage is set to a value in the neighborhood of the midpoint between the 0 and 1 levels of the eye pattern indicated by the waveforms of the electric signal, the ratio of 1 data to 0 data is 1:1.
Meanwhile, when the light intensity of the optical signal is high, the shot noise caused by the light receiving element is much greater than the heat noise generated by the amplifier. Therefore, the noise is greater at the 1 level of the eye pattern indicated by signal waveforms. When the threshold voltage is set to a value in the neighborhood of the 0 level of the eye pattern, the ratio of 1 data to 0 data is considered to be 1:1. However, the BER is adequately small even when the threshold voltage is set to a value in the neighborhood of the 1 level. Therefore, the receiving performance is satisfactory when the threshold voltage is set to a value in the neighborhood of the midpoint of the eye pattern.
The BER changes not only according to the light intensity of an optical signal, but also according to, e.g., waveform distortions due to the wavelength dispersion of an optical signal.
Again, the BER changes according to the light intensity or wavelength dispersion of an optical signal, or the like. The optimum threshold voltage as well changes according to the light intensity of optical signals, etc. However, the light receiving device as disclosed in Patent Literature 1 does not address the intensity of received light and the like.